Some codegen routines rename variables in the AST

[pramodk]

Current implementation of net_receive printing routine :

void CodegenCVisitor::print_net_receive_common_code(Block* node, bool need_mech_inst) {
    ....
    /// rename variables if they are actually used
    auto parameters = info.net_receive_node->get_parameters();
    if (!parameters.empty()) {
        int i = 0;
        printer->add_newline();
        for (auto& parameter: parameters) {
            auto name = parameter->get_node_name();
            VarUsageVisitor vu;
            auto var_used = vu.variable_used(node, "(*" + name + ")");
            if (var_used) {
                auto statement = "double* {} = weights + weight_index + {};"_format(name, i++);
                printer->add_line(statement);
                RenameVisitor vr(name, "*" + name);
                node->visit_children(&vr);
            }
        }
    }
}

If we are running second code generation pass (e.g. we were running OpenACC followed by C codegen), the weight is already renamed to weight*. And hence subsequent lookups will fail.

This was causing undefined weights in #101.

For now, temporary workaround is to activate one backend at a time (which is sufficient).